1. Field of the Invention
This invention relates generally to a three-terminal spin Hall effect magnetic-random-access memory (MRAM) element having a functional magnetic coupling layer and a spin hall effect base layer.
2. Description of the Related Art
In recent years, magnetic random access memories (hereinafter referred to as MRAMs) using the magnetoresistive effect of ferromagnetic tunnel junctions (also called MTJs) have been drawing increasing attention as the next-generation solid-state nonvolatile memories that can also cope with high-speed reading and writing. A ferromagnetic tunnel junction has a three-layer stack structure formed by stacking a recording layer having a changeable magnetization direction, an insulating tunnel barrier layer, and a fixed layer that is located on the opposite side from the recording layer and maintains a predetermined magnetization direction. Corresponding to the parallel and anti-parallel magnetic states between the recording layer magnetization and the reference layer magnetization, the magnetic memory element has low and high electrical resistance states, respectively. Accordingly, a detection of the resistance allows a magnetoresistive element to provide information stored in the magnetic memory device.
Typically, MRAM devices are classified by different write methods. A traditional MRAM is a magnetic field-switched MRAM utilizing electric line currents to generate magnetic fields and switch the magnetization direction of the recording layer in a magnetoresistive element at their cross-point location during the programming write. A spin-transfer torque (or STT)-MRAM has a different write method utilizing electrons' spin momentum transfer. Specifically, the angular momentum of the spin-polarized electrons is transmitted to the electrons in the magnetic material serving as the magnetic recording layer. According to this method, the magnetization direction of a recording layer is reversed by applying a spin-polarized current to the magnetoresistive element. As the volume of the magnetic layer forming the recording layer is smaller, the injected spin-polarized current to write or switch can be also smaller.
To record information or change resistance state, typically a recording current is provided by its CMOS transistor to flow in the stacked direction of the magnetoresistive element, which is hereinafter referred to as a “vertical spin-transfer method.” Generally, constant-voltage recording is performed when recording is performed in a memory device accompanied by a resistance change. In a STT-MRAM, the majority of the applied voltage is acting on a thin oxide layer (tunnel barrier layer) which is about 10 angstroms thick, and, if an excessive voltage is applied, the tunnel barrier breaks down. More, even when the tunnel barrier does not immediately break down, if recording operations are repeated, the element may still become nonfunctional such that the resistance value changes (decreases) and information readout errors increase, making the element un-recordable. Furthermore, recording is not performed unless a sufficient voltage or sufficient spin current is applied. Accordingly, problems with insufficient recording arise before possible tunnel barrier breaks down.
Reading STT MRAM involves applying a voltage to the MTJ stack to discover whether the MTJ element states at high resistance or low. However, a relatively high voltage needs to be applied to the MTJ to correctly determine whether its resistance is high or low, and the current passed at this voltage leaves little difference between the read-voltage and the write-voltage. Any fluctuation in the electrical characteristics of individual MTJs at advanced technology nodes could cause what was intended as a read-current, to have the effect of a write-current, thus reversing the direction of magnetization of the recording layer in MTJ.
It has been known that a spin current can, alternatively, be generated in non-magnetic transition metal material by a so-called Spin Hall Effect (SHE), in which spin-orbit coupling causes electrons with different spins to deflect in different directions yielding a pure spin current transverse to an applied charge current. Recently discovered Giant Spin Hall Effect (GSHE), the generation of large spin currents transverse to the charge current direction in specific high-Z metals (such as Pt, β-Ta, β-W, doped Cu) is a promising solution to the voltage, current scaling and reliability problems in a spin torque transfer MRAM.
Due to the thermal stability requirement, the recording layer is typically is patterned into an oval or ellipse like shape with an aspect ratio larger than 1.0 for a desired uniaxial shape anisotropy. A spin torque coming from the spin Hall effect has to be large enough to overcome a large energy barrier to switch the magnetization of the recording layer from one energy minimum state to the other energy minimum state, depending upon the spin Hall current direction.
Thus, it is desirable to provide a SHE STT-MRAM structure having much reduced switching energy barrier for recording while providing high enough thermal energy barrier for good data retention.